1. Field of the Invention
The invention relates to a method of discharging a picture display tube.
The invention also relates to a picture display device comprising a control circuit coupled to receive an off-command for supplying a switch-off signal and for supplying a video switching signal being the switch-off signal delayed by a given period, a video control circuit coupled to receive a picture signal and the video switching signal so as to apply in response thereto at least a one drive signal to at least one control electrode of a picture display tube for controlling a quantity of electrons in at least one electron beam independently of the picture signal, the quantity of electrons being substantially larger than zero, and a deflection circuit coupled to horizontal and vertical deflection means of the picture display tube for deflecting the electron beam. Discharging a picture display tube is important in all devices in which picture display tubes are used such as in, for example, television receivers and computer monitors.
2. Description of the Related Art
A picture display tube discharge circuit of this type is known from EP-A-0,521,378 corresponding to U.S. Pat. No. 5,184,225. The known picture display tube discharge circuit drives a picture display tube which comprises an anode for receiving an anode voltage, an electron gun for supplying an electron beam, horizontal and vertical deflection coils for deflecting the electron beam, and a display screen provided with a phosphor for converting the incident electron beam into light. The electron beam is accelerated towards the display screen by means of the anode voltage. The electron beam impinges upon the display screen at a position determined by deflection currents flowing in the horizontal and vertical deflection coils.
In response to a stand-by signal, the known picture display tube discharge circuit switches off a supply voltage of an anode voltage generator. Subsequently, an electron beam, consisting of a defined number of electrons (hereinafter referred to as electron current), is generated for causing the anode voltage to decrease to a low value. During the decrease of the anode voltage, power supply voltages supplied to horizontal and vertical deflection circuits also decrease and the surface area on the display screen written by the deflecting electron beam decreases. The area on the display screen written by the deflecting electron beam is further referred to as written area on the screen.
The quantity of energy reaching the display screen depends on the product of the electron current and the anode voltage and thus decreases with a decrease of the anode voltage. The known picture display tube discharge circuit ensures that such a quantity of electron current is applied that the anode voltage decreases at a sufficiently rapid rate with respect to the decrease of the horizontal and vertical deflection. The quantity of electron current and the rate at which the horizontal and vertical deflection decreases are chosen to be such that a written area on the screen has a decreasing size which is always sufficient to process the decreasing energy reaching the screen without phosphor burn-in.
The off-signal originates from a microcomputer and becomes active if a picture display device is switched from the normal operating state to the stand-by state. Discharging of the display tube as described above prevents burn-in of the phosphor and unwanted light-up of the display tube due to a high anode voltage, at decreased voltages on other electrodes of the display tube (cold emission), and also ensures that the anode voltage has decreased to a contact-safe value as is desirable in the case of servicing the device containing the picture display tube.
The existing display tube discharge circuit has the drawback that the defined electron current, generated to cause a decrease of the anode voltage, is visible on the display screen with a great brightness.
A safety circuit is known from GB 609,839. When an error is detected in a horizontal or vertical deflection circuit, this known safety circuit acts on the horizontal or vertical deflection so as to cause the electron beam to impinge upon the display screen outside the visible part of the screen. In this way, burn-in of the visible part of the phosphor is prevented. It is not an object of this safety circuit to discharge the display tube.